WASHINGTON (AP) -- Two senior Republican lawmakers said Thursday that terrorists are already changing their behavior after leaks about classified U.S. data gathering programs, but they offered no details.

Rep. Mike Rogers, R-Mich., said it's part of the damage from disclosures by National Security Agency contractor Edward Snowden of two NSA programs, which collect millions of telephone records and track foreign Internet activity on U.S. networks. Snowden fled to Hong Kong in May and has granted some interviews since then, saying he hopes to stay there and fight any charges that may yet be filed against him.

Rogers said there are "changes we can already see being made by the folks who wish to do us harm, and our allies harm" and that the revelations might also "make it harder to track bad guys trying to harm U.S. citizens in the United States."

Later Thursday, Sen. Saxby Chambliss of Georgia, the ranking Republican on the Senate Intelligence Committee, used similar language in criticizing Snowden.

"The bad guys are now changing their methods of operation," Chambliss said. "His disclosures are ultimately going to lead to us being less safe in America because bad guys will be able to figure out a way around some of the methods we use, and it's likely to cost lives down the road."

Rogers and Chambliss spoke after closed briefings with top administration officials on the matter.

The ranking Democrat on the committee, Rep. C.A. Dutch Ruppersberger of Maryland, said he's concerned that Snowden fled to Hong Kong, a part of China, "a country that's cyberattacking us every single day."

"It seems unusual that he would be in China and asking for the protection of the Chinese government ... but we're going to investigate," Ruppersberger said.

"He's obviously now decided that he wants to relay information about foreign-type (intelligence) collection," Rogers said. "Clearly, we're going to make a thorough scrub of what his China connections are," or whether he has a connection to any other foreign government, the congressman added.

The NSA's director, Gen. Keith Alexander, who was part of the closed briefings to Senate and House members, said he hopes to declassify details of dozens of attacks disrupted by the programs. Alexander said officials don't want to "cause another terror attack by giving out too much information."

Officials have thrown out widely varying numbers of the attacks they say the broad surveillance of Americans' phone and online usage has thwarted. On Wednesday, Alexander said dozens have been stopped. Ruppersberger said the surveillance "has thwarted 10 possible terrorist attacks," then amended that number to be in line with Alexander's statement. In the initial days after the disclosures of the programs, officials cited one case.

Two senators and longtime critics of the program challenged Alexander's claim Thursday.

"We have not yet seen any evidence showing that the NSA's dragnet collection of Americans' phone records has produced any uniquely valuable intelligence," Sens. Mark Udall, D-Colo. and Ron Wyden, D-Ore., both members of the Senate Intelligence Committee, said in a statement. "All of the plots that he mentioned appear to have been identified using other collection methods."

The disclosures raised privacy concerns as Americans — some of them members of Congress — learned for the first time the extent of surveillance powers granted by the secret Foreign Intelligence Surveillance Court to help U.S. intelligence and law enforcement agencies track terrorists.

Investigators have been trying to determine which facilities the 29-year-old Snowden visited during his intelligence career to decide how much classified data he had access to as a computer systems analyst for the NSA and earlier for the CIA, according to two congressional staffers. They spoke on condition of anonymity because they were not authorized to describe the investigation publicly.

"It's clear he attempted to go places he was not authorized to go," within the classified systems, Rogers said. He called Snowden "a fairly low-level individual, but because of his position in the IT system had access to certain pieces of information that, candidly, he did not understand, or had the full scope of what these programs where, who decided on his own he was going to release this information."

Snowden's access to secret programs is spurring lawmakers to consider imposing new limits on contractors who work in the intelligence field. The head of the Senate Intelligence Committee, Dianne Feinstein, D-Calif., said Thursday that her committee would draft legislation to limit or prevent contractors from handling highly classified technical data.

Feinstein spoke after a closed-door briefing Thursday on the NSA leaks open to all senators, by officials including Alexander and Director of National Intelligence James Clapper, and FBI Deputy Director Sean Joyce. Feinstein said 47 attended — almost half the Senate.

FBI Director Robert Mueller defended the programs in testimony to Congress on Thursday. In what is likely his final appearance as FBI director before the House Judiciary Committee, Mueller said that terrorists track leaked information "very, very closely" and that because of leaks "we lose our ability to get their communications" and "we are exceptionally vulnerable."

Rep. John Conyers of Michigan, the Judiciary Committee's ranking Democrat, said, "It's my fear that we are on the verge of becoming a surveillance state."

In defending the programs, Mueller called attention to the run-up to the 2001 terrorist attacks, saying that if the controversial surveillance efforts had been in place back then, they might have uncovered the hijackers' plot. The 9/11 Commission found that among the major U.S. failures before the attack was that agencies didn't share information they already had about suspected terrorists with the FBI.

"If we had had this program, that opportunity would have been there," Mueller said.

"I am not persuaded that that makes it OK to collect every call," Conyers replied.